A skid-steer vehicle is self-propelled vehicle which has drive wheels or tracks on two opposed sides of the vehicle and which achieves a steering function by controlled rotation of one of the opposed drive wheels or tracks relative to the other. Relative rotation between the opposed drive wheels or tracks results in a change in direction of the vehicle and so provides a steering function. With such vehicles it is important that the drive wheels or tracks each have a tread formation that provides good traction with the ground and good directional control both in straight-line travel and in turning.